Illegal copy of content such as a computer program, an image, and music (hereinbelow, collectively called “content”) without proper purchase procedures has become a problem. As methods of preventing the illegal copy, the method is already known that a copy preventing function or a limitation of the number of copying times is provided to a proper product. On the other hand, a billing system is also known, in which copying is regarded as “distribution” and content copied without proper purchase procedures can be used only after performing predetermined billing process (see, for example, International Published Patent Application WO 01/033546).